Air separation is a very power intensive technology, consuming thousands of kilowatts or several megawatts of electric power to produce large quantities of industrial gases for tonnage applications such as chemicals, refineries, steel mills, etc.
A typical liquid pumped process is illustrated in FIG. 1. In this type of process, atmospheric air is compressed by a Main Air Compressor (MAC) 1 to a pressure of about 6 bar absolute, it is then purified in an adsorber system 2 to remove impurities such as moisture and carbon dioxide that can freeze at cryogenic temperature to yield a purified feed air. A portion 3 of this purified feed air is then cooled to near its dew point in heat exchanger 30 and is introduced into a high pressure column 10 of a double column system in gaseous form for distillation. Nitrogen rich liquid 4 is extracted at the top of this high pressure column and a portion is sent to the top of the low pressure column 11 as a reflux stream. The oxygen-enriched liquid stream 5 at the bottom of the high pressure column is also sent to the low pressure column as feed. These liquids 4 and 5 are subcooled before expansion against cold gases in subcoolers not shown in the figure for the sake of simplicity. An oxygen liquid 6 is extracted from the bottom of the low pressure column 11, pressurized by pump to a required pressure then vaporized in the exchanger 30 to form the gaseous oxygen product 7. Another portion 8 of the purified feed air is further compressed in a Booster Air Compressor (BAC) 20 to high pressure for condensation in the exchanger 30 against the vaporizing oxygen enriched stream. Depending upon the pressure of the oxygen rich product, the boosted air pressure can be around 65 bar or sometimes over 80 bar. The condensed boosted air 9 is also sent to the column system as feed for the distillation, for example to the high pressure column. Part of the liquid air may be removed from the high pressure column and sent to the low pressure column following subcooling and expansion. It is also possible to extract nitrogen rich liquid from the top of the high pressure column then pump it to high pressure (stream 13) and vaporize it in the exchanger in the same way as with oxygen liquid. A small portion of the feed air (stream 14) is further compressed and expanded into the column 11 to provide the refrigeration of the unit. Optionally alternative or additional means of providing refrigeration may be used, such as Claude expanders or nitrogen expanders.
Waste nitrogen is removed from the top of the low pressure column and warms in exchanger 30. Argon is produced using a standard argon column whose top condenser is cooled with oxygen enriched liquid 5.
A typical 3,000 ton/day oxygen plant producing gaseous oxygen under pressure for industrial uses can consume typically about 50 MW. A network of oxygen plants for pipeline operation would require a power supply capable of providing several hundreds megawatts of electric power. In fact, the electric power is the main operating cost of an air separation plant since its raw material or feedstock is atmospheric air and is essentially free. Electric power is used to drive compressors for air or products compression. Therefore, power consumption or process efficiency is one of the most important factors in the design and operation of an air separation unit (ASU). Power rate, usually expressed in $/kWh, is not constant during the day but varies widely depending upon the peaks or off-peaks. It is well known that during the day the power rate is the highest when there is strong demand—or peak period—and the lowest during the low demand—or off-peak period. Utility companies tend to offer significant cost reduction if an industrial power user can cut back its power consumption during peaks. Therefore, the companies operating air separation units always have strong incentives to adjust the operating conditions of the plants to track the power demand so that to lower the utility cost. It is clear that a solution is needed to provide an economical answer to this variable power rate issue.
It is useful to note that the periods when the power peaks take place may be totally different from the product demand peaks, for example, a warm weather would generate a high power demand due to air conditioning equipment meanwhile the demand for products remains at normal level. In several locations, the peaks occur during the day time when the industrial output of manufacturing plants, the main users of industrial gases, is usually at the highest level and when combined with the high power usage of other activities would cause very high demand on the electric grid. This high power usage creates potential shortage and utility companies must allocate other sources of power supply causing temporary high power rate. Also, usually at night, the power demand is lower and the power is available abundantly such that the utility companies could lower the power rate to encourage usage and to keep the power generating plants operate efficiently at reduced load. The power rate at peaks can be twice or several times higher than the power rate for off-peaks. In this application, the term “peak” describes the period when power rate is high and the term “off-peak” means the period when power rate is low.
For industrial power users, power rates are usually negotiated and defined in advance in power contracts. In addition to the daily variation of power rates, sometimes there are provisions or allowances for interruptible power supply: during periods of high power demand on the power grid, the utility companies can reduce the supply to those users with a relatively short advance notice, in return, the overall power rate offered can be significantly below the normal power rate. This kind of arrangement provides additional incentives for users to adapt their consumption in line with the network management of the power suppliers. Therefore, significant cost reduction can be achieved only if the plant equipment can perform such flexibility. Based on the power cost structure as set forth by the power contracts, the users can define predetermined threshold or thresholds of power rate to trigger the mechanism of power reduction:                when power rate is above the predetermined threshold, the power usage is reduced to lower the cost.        when power rate is below the predetermined threshold, the power usage is increased to normal level or even higher if desired.        
A simple approach to address the problem of variable power rate is to lower the plant's power consumption during peaks while maintaining the product output in order to satisfy the customer's need. However, the cryogenic process of air separation plants is not very flexible since it involves distillation columns and the product specifications require fairly high purities. Attempts to lower the plant output in a very short time or to increase the plant production quickly to meet product demand can have detrimental effects over plant stability and product integrity. Various patents have been written to suggest how to solve the difficulties associated with the variable product demand of a cryogenic plant.
U.S. Pat. No. 3,056,268 teaches the technique of storing oxygen and air under liquid form and vaporizing the liquids to produce gaseous products to satisfy the variable demand of the customer, such as at metallurgical plants. The liquid oxygen is vaporized when its demand is high. This vaporization is balanced by a condensation of liquid nitrogen via the main condenser of the double column air separation unit.
U.S. Pat. No. 4,529,425 teaches a similar technique to that of U.S. Pat. No. 3,056,268 to solve the problem of variable demand, but liquid nitrogen is used instead of liquid air.
U.S. Pat. No. 5,082,482 offers an alternative version of U.S. Pat. No. 3,056,268 by sending a constant flow of liquid oxygen into a container and withdrawing from it a variable flow of liquid oxygen to meet the requirement of variable demand of oxygen. Withdrawn liquid oxygen is vaporized in an exchanger by condensation of a corresponding flow of incoming air.
U.S. Pat. No. 5,084,081 teaches yet another method of U.S. Pat. No. 4,529,425 wherein another intermediate liquid, the oxygen enriched liquid, is used in addition to the traditional liquid oxygen and liquid nitrogen as the buffered products to address the variable demand. The use of enriched oxygen liquid allows stabilizing the argon column during the variable demand periods.
In still another approach to address the variable product demand, U.S. Pat. No. 5,666,823 teaches a technique to efficiently integrate the air separation unit with a high pressure combustion turbine. Air extracted from the combustion turbine during the periods of low product demand is fed to the air separation unit and a portion is expanded to produce liquid. When product demand is high, less air is extracted from the combustion turbine and the liquid produced earlier is fed back to the system to satisfy the higher demand. The refrigeration supplied by the liquid is compensated by not running the expander for lack of extracted air from the combustion turbine during the high product demand.
The above publications addressed the technical issues of the variable demand, especially the techniques used to maintain stability of the distillation columns during the time when the demand of the product varies widely. However, none of the above directly address the aspect of potential savings and economy when adapting the air separation plants to the power rate structure of peak and off-peak periods to obtain cost reduction. Industry practice also does not resolve the technical problems associated with the adjustment of the air separation units during periods of high power cost and with relatively unchanged product demand. In fact, these two aspects of the operation of air separation units are quite different by nature: one is governed by the customer's variable demand and the other is governed by variable power cost with relatively constant demand.
Therefore, there exists a need to come up with a configuration for air separation plants permitting a reduction of the power consumption during peaks, while maintaining a supply of products to satisfy customer's demand. To make up for this reduction of power, additional power consumption can be arranged to take place during off-peak periods, at a much lower power rate. Significant savings on power rate can therefore be achieved, since a portion of the products is being produced at a low power rate and supplied to the customers during periods of high power rate.